1. Field of the Invention
The present invention relates to a process to prepare TiO.sub.2 pigment involving adding both a first oxide precursor, which is a silicon halide such as SiCl.sub.4, and at least one second oxide precursor to a gaseous suspension comprising TiO.sub.2 particles. The second oxide is selected from oxide precursors of boron, phosphorus, magnesium, niobium, germanium, and mixtures thereof.
2. Description of the Related Art
The process for producing titanium dioxide (TiO.sub.2) pigment by reacting an oxygen-containing gas and titanium tetrachloride (TiCl.sub.4) at temperatures ranging from 900.degree. to 1600.degree. C. in a vapor phase is known. The resulting hot gaseous suspension of TiO.sub.2 particles and free chlorine are discharged from the reactor and must be quickly cooled in a conduit, i.e., a flue, so that undesired TiO.sub.2 particle size growth is prevented and particle agglomeration is minimized.
Titanium dioxide is widely used as a white pigment. In many commercial applications, such as paints, high durability is required, durability meaning the ability of the pigment to withstand the destructive effects of weather, most importantly, sunlight. Since TiO.sub.2 is photoactive, and can promote degradation of paint systems, hydrous oxide coatings are applied to TiO.sub.2 particles to improve durability. Typically hydrous oxide coatings on TiO.sub.2 particles are prepared by wet chemical methods. These involve precipitation of the hydrous oxide, such as silica, alumina, zirconia, from solution. While these processes do provide somewhat durable coatings on the TiO.sub.2 particles, they often result in uneven, non-uniform and porous coatings. These processes also often require milling of the pigment prior to the wet coating methods to break up soft aggregates to assure all particles are coated. Another problem with these processes is that they tend to provide coated TiO.sub.2 particles which have low gloss. Further, these processes require substantial investment in equipment, involve time consuming, often complicated operations, and generate volumes of aqueous wastes.
It would therefore be desirable to have a process for making a durable TiO.sub.2 pigment having particles with a dense uniform coating. It would also be desirable to reduce investment costs involved in a coating process, while minimizing waste. A high percentage of the TiO.sub.2 particles should be coated by such a process, as any uncoated particles may result in a pigment having significantly lower durability. The present invention provides such a process.
Gonzalez, U.S. Pat. No. 5,562,764 discloses a process for the preparation of TiO.sub.2, whereby a volatile silicon compound is added downstream of where the oxygen-containing gas and TiCl.sub.4 are initially contacted to produce a substantially anatase-free TiO.sub.2 pigment containing silica. The TiO.sub.2 product has decreased particle size and agglomeration.
Kodas, et al., WO 96/36441, disclose a process to prepare coated TiO.sub.2 pigment comprising oxidation of a titanium compound to form TiO.sub.2 particles in a reactor, subsequently introducing at least one metal coating precursor into the reactor and oxidation of the precursor to form a metal oxide coating on the TiO.sub.2 pigment particles. The metal oxide is selected from SiO.sub.2, Al.sub.2 O.sub.3, ZrO.sub.2 and mixtures thereof.
Allen and Gergely in co-pending and co-assigned U.S. patent application Ser. No. 08/764,414, filed Dec. 11, 1996, now allowed, disclose a process to prepare rutile TiO.sub.2 which comprises oxidation of TiCl.sub.4 in the presence of an aluminum compound to produce a suspension of TiO.sub.2 particles and then contacting the particles with a boron compound to provide a TiO.sub.2 pigment containing B.sub.2 O.sub.3. The TiO.sub.2 product has decreased particle size and agglomeration, and less abrasion than conventional rutile TiO.sub.2 pigments.
Glaeser, U.S. Pat. No. 4,214,913, discloses a process to prepare rutile TiO.sub.2 which comprises oxidation of TiCl.sub.4 in the presence of AlCl.sub.3 and addition of PCl.sub.3 into the oxidation at a point when at least 80% of the TiCl.sub.4 has been converted to TiO.sub.2. The TiO.sub.2 product is provided in a state which can be directly used to prepare slurries for use in paper and board applications. It is stated that for use in coatings, the TiO.sub.2 is wet treated.
Angerman and Moore, U.S. Pat. No. 3,856,929 disclose a process to prepare anatase TiO.sub.2 which comprises oxidizing TiCl.sub.4 in the presence of phosphorus and silicon halides. Adding silicon and phosphorus halides in combination is more effective than either halide alone at promoting formation of anatase TiO.sub.2.
There is a need to provide a substantially rutile TiO.sub.2 pigment having durability and gloss. There is also a need to reduce or eliminate problems associated with conventional wet treatment methods used to provide coatings on TiO.sub.2 particles.
The present invention meets the foregoing needs.